Field of the Invention
This invention relates to large-mode-area (LMA) fibers designed to compensate for the effects of bends in the fiber and to suppress higher-order modes (HOMs) and, more particularly to high power optical amplifiers that incorporate such LMA fibers.
Discussion of the Related Art
Optical fiber amplifiers have great impact in diverse applications ranging from high power devices used for cutting, welding and range finding to lower power devices used to amplify optical carrier signals in telecommunication systems. In the former case, the high power amplifier utilizes a gain-producing fiber (GPF; e.g., a LMA fiber doped with suitable rare-earth species or chromium) and a source of pump light to amplify signal light propagating in effectively a single mode (i.e., the fundamental transverse mode) through the LMA fiber.
LMA fibers, which have a relatively large effective area (Aeff), are used to reduce optical power density and, therefore, also reduce optical nonlinearities in the fiber. However, larger area fibers typically support several or many modes, increasing the likelihood that HOMs will also propagate in the fiber and undergo amplification, thereby degrading the quality of beam. Beam quality is often characterized in terms of a parameter known as M2 
(M2=1 for an ideal Gaussian beam), whereas single modedness can be characterized by various techniques including spatially and spectrally (S2) resolved imaging, as described by Nicholson et al., Optics Express, Vo. 16, No. 10, pp. 7233-7243 (2008), which is incorporated herein by reference. Bends in the fiber exacerbate this problem they reduce the ability of various fiber designs to selectively suppress HOMs while ensuring propagation of the fundamental mode at power levels that satisfy typical performance requirements.
In a typical conventional amplifier configuration a few meters (e.g., 5 m) of GPF is coiled within an amplifier package that may also contain other components of the amplifier. In some designs, those components include a non-GPF LMA (e.g., a fiber pigtail) optically coupled to the GPF. Coiling the LMA fiber, an expedient to save space, means that the fiber is bent.
Bends in the LMA fiber are a key factor imposing performance tradeoffs between three principal goals of LMA fiber design: large mode area, low loss, and single-mode operation. Macrobend loss is often the dominant source of loss, bend distortion limits the scaling of area, and bends degrade single-mode operation, as noted above, by limiting the degree to which unwanted HOMs can be selectively suppressed.
One strategy for ameliorating the adverse effects of bending is to pre-compensate the refractive index profile of an unbent (as-fabricated, straight) fiber for the expected bend-induced perturbation, as described by Fini, Opt. Express, Vol. 14, No. 1, pp. 69-81 (2006), which is incorporated herein by reference. This strategy, which utilizes an asymmetric index profile, has been exploited by others in the design of bend-compensated microstructure fibers. [See, for example, Minelly, U.S. Pat. No. 7,876,495 (2011), which is also incorporated herein by reference.] However, this strategy may be difficult to implement. It requires an asymmetric index profile across the fiber cross-section, and it requires deployment of the fiber in a fixed azimuthal orientation throughout the bend.
By the terms unbent, straight, and as fabricated we mean the bend radius of the fiber is essentially infinite (a perfectly straight fiber) or so large that any resulting bend would have an insignificant effect on the fiber performance for the intended application of the fiber.
As noted in the aforesaid parent application (Fini 21-31), there is a need for an LMA fiber design that provides (i) bend compensation without requiring an asymmetric index profile (either asymmetric or symmetric index profiles are suitable); and (ii) HOM suppression in addition to compensation.
These needs are addressed by the designs described below. The analysis of bends in LMA fibers, as described in the parent application, has uncovered a surprising result the selectivity of HOM suppression is degraded primarily by the bend perturbation of the inner cladding region, not by perturbation of the core region or other regions of the fiber. Thus, to dramatically improve the basic performance tradeoff it is sufficient to compensate the bend perturbation in the inner cladding region. Unlike the prior art, bend compensation accompanied by sufficient HOM suppression requires neither asymmetry of the index profile of the core region nor asymmetry of the index profile across the entire fiber cross-section. However, the principal design features of our invention do not exclude the use of asymmetric index profiles either.
In accordance with a first aspect of the invention described in the parent application, a bend-compensated optical fiber comprises a core region having a longitudinal axis and a cladding region surrounding the core region. The core and cladding regions are configured to support and guide the propagation of signal light in a fundamental transverse mode in the core region in the direction of the fiber axis. The cladding region includes an inner cladding region surrounding the core region and an outer cladding region surrounding the inner cladding region. At least a longitudinal segment of the fiber is configured to be bent or coiled to a bend radius Rb. (Bending changes a gradient of the index profile of a straight fiber, producing what is known in the art as an equivalent index profile within the bent segment.) At least the longitudinal fiber segment is pre-compensated in that (i) the transverse cross section of the fiber has a refractive index profile that is approximately azimuthally symmetric with respect to the fiber axis and (ii) the refractive index of at least a transverse portion of the inner cladding region is graded with a slope configured to compensate for the expected change of the gradient in the index profile that would be induced by the bend; that is, to compensate for the expected equivalent index profile.
In one embodiment of the first aspect of the invention described in the parent application, bend compensation of our LMA fiber, with excellent HOM suppression to enable effectively single-mode operation, is achieved by grading the refractive index of the inner cladding region of the LMA fiber, preferably with a slope γncore/Rb, where γ falls in the range 0.6-1.2; where γ=1 corresponds to ideal compensation assuming the well-known geometrical conformal mapping [see, Marcuse, Appl. Opt., Vol. 21, p. 4208 (1982), which is incorporated herein by reference.], but preferred designs may include a stress correction (e.g., γ=0.8) or other adjustments that allow for curvature variations within a coil, etc; ncore is the index of the core region; and Rb is the bend radius. [Regarding, bend-induced strain (stress), see, Nagano, Applied Optics Vol. 17, No. 13, pp. 2080-2085 (1978), which is also incorporated herein by reference.]
In another embodiment of the first aspect of the invention described in the parent application, the inner cladding region is annular having an inner radius r1 and an outer radius r2 such that the ratio r2/r1 is configured to suppress the propagation of HOMs. In some embodiments we prefer r2/r1>2 and in others we prefer r2/r1>3 depending on the desired level of HOM suppression.
In accordance with a second aspect of the invention described in the parent application, a bend-compensated optical fiber comprises a core region having a longitudinal axis and a cladding region surrounding the core region. The core and cladding regions are configured to support and guide the propagation of signal light in a fundamental transverse mode in the core region in the direction of the fiber axis. The cladding region includes an inner cladding region surrounding the core region and an outer cladding region surrounding the inner cladding region. At least a longitudinal segment of the fiber is configured to be bent or coiled to a bend radius Rb and at least the longitudinal segment is pre-compensated in that the refractive index of at least a transverse portion of the inner cladding region is graded with a slope configured to compensate for the expected gradient change in the index profile that would be induced by the bend; that is, to compensate for the equivalent index profile. In addition, the inner cladding region is annular having an inner radius r1 and an outer radius r2 such that the ratio r2/r1 is configured to suppress the propagation of HOMs. In some embodiments of the second aspect of our invention, we prefer r2/r1>2 and in others we prefer r2/r1>3 depending on the desired level of HOM suppression.
In some embodiments of the second aspect of the invention described in the parent application, at least a longitudinal segment of the fiber is configured to be bent or coiled to a bend radius Rb and within the segment the transverse cross section of the fiber has a refractive index profile that is approximately azimuthally symmetric with respect to the fiber axis. These symmetric embodiments enable the fiber to be deployed without requiring a fixed azimuthal orientation.
In other embodiments of the second aspect of the invention described in the parent application, at least a longitudinal segment of the fiber is configured to be bent or coiled to a bend radius Rb and within the segment the transverse cross section of the fiber has a refractive index profile that is asymmetric with respect to the fiber axis.